The basic concept of this challenge is that you have four sisters, which I have represented here with the four elements(but you don't have to call them that), who require ambrosia to prolong their everlasting youth and who want to improve the town by bearing and raising as many children as possible.

You start by creating four sisters at the young adult lifestage with these traits(you can choose their appearance, name, and LTW to be anything you want):
Fire: flirty, great kisser, charismatic, friendly, dislikes children
Water: neat, natural cook, handy, family oriented, perfectionist
Air: bookworm, genius, clumsy, absent minded, loner
Earth: angler, green thumb, loves the outdoors, athletic, disciplined(if you have WA) OR vegetarian(if not).
Here are the roles of the sisters:
Fire: The only sister who is allowed to leave the lot freely. Her job is to search out men, develop relationships with them, and bear their children. She is not allowed to marry, nor are the men allowed to move in. She may bring one man home each night and may try for baby once every 24 hours. Additional woohoo(same and opposite sex) may take place as often as necessary. She also teaches music lessons when the children are older. Skills which she is allowed to develop and use are charisma, music and art.
Water: She is the caretaker of the young children and of the house. Cooking, cleaning and repair duties fall to her, as well as the care of young children. Her responsibilities for the children include teaching the three basic toddler skills, feeding, socializing and changing diapers of babies and toddlers, and playing with toddler toys. The only skill which she is allowed to develop and use is cooking.
Air: She is the keeper of knowledge. Once children become old enough to learn, she is responsible for teaching them. She reads toddler books to the very young children, and as they get older she helps them with homework and tutors them in various skills. She is allowed to learn photography, nectar-making, martial arts, cooking, handiness, charisma, gardening and logic from books only.
Earth: She is the gardener and athletics master. She is responsible for growing food to sell and eat, fishing for fertilizer or ingredients, as well as training children in athletics and martial arts. She is also the only sister(apart from the fire sister) who is allowed to leave the lot. She is allowed to visit the grocery store to purchase seeds, or what food she can't grow. Skills which she is allowed to develop and use are gardening, fishing, athletics and martial arts(if you have WA).

You may use motherlode or kaching as much as necessary while building your initial lot. You may also place as many lighting, or decoration(plants, clutter, or other objects that have no use other than looking nice and raising environment score) objects as you want. After you have obtained these items, all travel restrictions go into effect and you must get your on-hand cash down to 10k. That should be plenty of money, but you can supplement it during the course of the challenge by selling works of art that the fire sister or the children create, or fish/plants that the earth sister obtains.

Object of the challenge: To have as many children as possible while ensuring that they never fail a lifestage(I will say what the criteria for that are).
Scoring: 1 point per child who grows to young adult and moves out. 1 additional point if the fire sister paints a "portrait" of them(not a still life) as a young adult before they move out, and keeps it on display on the lot.
Ways to lose:
If you fail any life stage.
If any of the sisters or the children die.
If any of the sisters age up to Adult from Young Adult.
If any children are taken away by the social worker.
If the repo man comes.

Responsibilities at each life stage:
Pregnant:
The fire sister may not leave the lot
The only responsibility is to keep her happy and comfortable.
Fail if: She leaves the lot or the baby is born with random 'bad' traits(ie, if she had a bad mood through the pregnancy).

Infant:
Keep the baby fed, clean, and socially happy.
Fail if: You can't fail this, or if you have you've also violated another lose condition.

Toddler:
Toddlers must be taught all three basic skills from the water sister.
They also must max out the three levels on the two skill toys, which they can do with or without the water sister's help.
In addition, the air sister must read all toddler books to them(they can't read them to themselves).
Fail if: The toddler ages up without learning all three basic skills, the toddler ages up without maxing all skill objects(3 levels each on the logic and music toys, and all books read to them), or if the toddler ages up with less than 50% relationship with the water sister.

Child:
The child must attend school and do homework to work towards an A.
They must also skill up on the play table and toy oven, until they've maxed the skill levels.
They should do homework or tutor their skills with the air sister.
Skills to concentrate on at this life stage are logic and art.
They must also develop a relationship with the earth sister, though games or talking, even if there's nothing she can teach them yet.
Fail if: The child misses a day of school, the child ages up with less than an A in school, the child ages up with less than 50% relationship with the air sister, the water sister and the earth sister, the child ages up without maxing all levels on the play table(3 levels of handiness) and the toy oven(3 levels of cooking), or the child ages up with less than the following skills â€“ logic 5, art 5.

Teen:
By the end of the first week as a teen, they must have a BFF.
They must find a boyfriend/girlfriend of their choice, and go steady with them.
They also must continue to keep up with school and homework.
They begin music lessons with the fire sister. The single guitar may only be placed in her quadrant, and is not allowed to be moved. Just pretend that she is teaching the teen â€“ you can have her â€śwatchâ€ť them play to raise the relationship level.
They should also help the earth sister with her garden, and receive some athletic or martial arts training from her as well.
Fail if: The teen misses a day of school, the teen ages up with less than an A in school, the teen ages up with less than 90% relationship with their BFF or BF/GF, the teen ages up with less than 50% relationship with ANY sister, the teen ages up with less than the following skills â€“ logic 5, art 5, music 5, cooking 3, handiness 3, gardening 3, martial arts 3(if you have WA) OR athletic 3(if not).

Additional rules:
No cheats.
You may not purchase additional items from buy mode.
You must use normal lifespan.
Nobody is allowed to get a job(full- or part-time)
You can pick the traits of the children, but no 2 children may have the same combination of traits. For example: child 1 is athletic, artistic, clumsy, handy and hot-headed. If child 2 is artistic, clumsy, handy and hot-headed, they can't pick athletic as their 5th trait.
You may not accept opportunities.

It's a bit of a complicated challenge idea, so let me know if you have any questions or if I've forgotten to explain something important. I hope somebody else out there finds it entertaining.

Sounds fun but a singel question or two.
Is it allowed that the two sisters who cant leave the lot goes just over the street to get rid of that nasty crazy modlet??
Can they go around in the whole house or do they have to be in their own section only??

Im going to make my sisters and if i remember it post the photos of them but i cant play them before ive finnished some other challenges.

This sounds fun! If any of the sisters but the water sister have to use the bathroom or take a shower, do they have to go over to the water sisters part? Or can they have a toilet and a bathtub in their own part?
Edit: Can we also put an easel in the fire sisters part? Because you mention creating portraits...

So far, very fun. Questions: When does this challenge end? And by the description I'm assuming we should also buy the kid stove and art table. I'll wait until I get the okay before buying them.

Let me introduce my sisters:

Here's Rei, my fire sim.

Ami, my air sim.

Makoto, my earth sim.

And Michiru, my water sim.

And here are their humble abodes.

Rei has had her first child with Hank Goddard and everyone is about halfway through their primary skills and two-three into their secondary skills(with the exception of Makoto, who has slightly higher cooking and no secondary skills).

I'd like to know when the challenge ends and what mods are allowed. Right now, I am running on no mods other than the don't put away fire pit mod.

Some questions and answers...a lot of things I hadn't considered, or had forgotten to put in the original post! Nice screenshots, That20issmokin. I didn't realize it was even possible to fit all the water sister things in one floor, I have it split between three in my game. I'll post screenshots after I play tomorrow.

Is it allowed that the two sisters who cant leave the lot goes just over the street to get rid of that nasty crazy modlet?? I'd say no, but you should accumulate enough lifetime aspiration points over the course of the challenge that it should be trivial for one of the sisters to purchase the moodlet manager, and you can use that to remove the stir crazy moodlet.

Can they go around in the whole house or do they have to be in their own section only?? Anybody can visit any other section of the house.

This sounds fun! If any of the sisters but the water sister have to use the bathroom or take a shower, do they have to go over to the water sisters part? Or can they have a toilet and a bathtub in their own part? All of the bathroom objects exist in the water sister's part, because it only makes sense that way. The sole exception is the sink in the fire sister's area, because I believe you need one to clean up plates of drinks that you make from the bar, and it seemed like a hike to take them the whole way across the lot.

Can we also put an easel in the fire sisters part? Because you mention creating portraits... Uh, yeah...can't believe I forgot that... >_>

when does the challenge end? When you run out of people to have babies with(because they're all descended from you), when you run out of valid trait combinations, when you lose by having one of the loss conditions occur, or when you just get tired of playing.

what mods are allowed As long as it doesn't give you an unfair advantage over others playing, it's allowed. Fast skill mods are out, as are mods that would make plants grow overnight, or allow you to purchase ambrosia from the store, make group servings of ambrosia, etc. Of course, if you want to make your game harder, knock yourself out.

I'm assuming we should also buy the kid stove and art table. Ah, sorry about that. They were in my house, but I forgot to add them to the list.

So is our fire gal the only one allowed to have children, or can the others do so as long as they don't leave the lot? Only the fire sister can have children. Make up your own story for why this is. :p

I think it would be nice if the watersister could make nectar to that would fit i think.
And let the air sister learn all the skill then she wants to be as genius as possible in my mind.
Maybe the fire sister could learn the photography skill to.

And i would like if the fire sister for every 5 kid she gives birth to or so could take a trip to france, egypt or china. And take her earth sister with her for every tenth child or so.

I deffintly going to play this challenge - but i think i would put in the things ive mentioned above just to keep meself satisfied.

I agree with your view on the air sister being a genius, and wanting to learn everything. I gave her all the skills that can be learned by book-learning, leaving out things like guitar, painting, fishing and athletic where she has to actually use objects, because I felt that learning from books fit her better.

I feel kind of weird about the idea of the sisters going on vacations, but the idea of content unlocking after a certain number of children raised is interesting. I might write that into the challenge. Is there anything else that might be interesting to be able to do, that isn't already in the challenge, beyond nectar-making and photography?

Quick Question about this: "You may also place as many lighting, or decoration(plants, clutter, or other objects that have no use other than looking nice and raising environment score) objects as you want."

Can this include end tables? Just as a means to put lamps/flowers etc... on? I went ahead and put some in seeing as they serve no apparent purpose other than that.

@smashersaurus: *grabby hands* I can has your house? It is sooo beautiful and to be honest I'm kind of dreading making the house for my sisters. :< I especially love the earth one.

Two things: I changed the fire sister's last trait from "dislikes children" to "hot-headed," as I felt it fit the fire thing better. It'll probably make the game a little less challenging, but oh well. Also, I couldn't find a LTW the air sister could fulfill without breaking the rules. :/ I went with the one about being a best-selling novelist. What's your feeling on the air sister writing novels?

@tairourou - You can randomize the LTWs by going into the trait selection window and going out again. With the trait build I gave above, she has 2-3 that stay the same, and the rest are random. I just randomized them until she rolled one I could do(renneisance(sp?) sim). It would be a bit difficult for her to write novels, because a computer isn't allowed on the lot(I hate my sims becoming loser couch/desk potatos, so I like to make houses without computers or tvs).

@tairourou - You can randomize the LTWs by going into the trait selection window and going out again. With the trait build I gave above, she has 2-3 that stay the same, and the rest are random. I just randomized them until she rolled one I could do(renneisance(sp?) sim). It would be a bit difficult for her to write novels, because a computer isn't allowed on the lot(I hate my sims becoming loser couch/desk potatos, so I like to make houses without computers or tvs).

I think there's a mod or cheat code to give ltw points to your sim. You can cheat to give yourself enough to buy the midlife crisis reward, and change it that way.
Alternately, if you use awesomemod(or another mod that allows you to go back to cas), you can try editing them that way, but I don't know if changing the ltw/traits that way will cause everything to explode or not as I've never tried.